The present invention relates generally to coherent receiver signal matching, and more particularly, to the self alignment of the relative phase and amplitude of two coherent receivers.
Matched coherent receiver channels are often required in radar and communications work in order to compare responses from different sensors or to coherently downshift in frequency and detect signal components. For example, two receiver channels of a coherent quadrature detector system (sometimes called a single sideband demodulator) are typically used to frequency translate signals received within a nominal I.F. frequency band down to two separate baseband signal components which together contain the original signal's phase and amplitude information.
If it is assumed that the input I.F. signal is a sinewave represented by S.sub.I.F. =A cos (2.pi.f.sub.1 T+.theta.), then a quadrature detector circuit with a local oscillator input of S.sub.L.O. =L sin 2.pi.f.sub.2 T will produce the signal components represented as EQU I=.alpha..sub.I A sin [2.pi.(f.sub.1 -f.sub.2) T+.phi..sub.I +.theta.] EQU Q=.alpha..sub.Q A cos [2.pi.(f.sub.1 +f.sub.2) T+.phi..sub.Q +.theta.],
with the .alpha. terms representing circuit gain or attenuation values and the .phi. terms representing circuit phase shift and delay terms. For the ideal matched case, .alpha..sub.I =.alpha..sub.Q =.alpha. and .phi..sub.I =.phi..sub.Q =.phi. such that EQU I=-.alpha. A sin [2.pi.(f.sub.1 -f.sub.2) T+.phi.+.theta.] EQU Q=.alpha. A cos [2.pi.(f.sub.1 -f.sub.2) T+.phi.+.theta.]
The desired amplitude A of the signal may be found by the following equation: EQU .alpha. A=(I.sup.2 +Q.sup.2).sup.1/2.
Likewise, the phase of the I.F. signal may be calculated by the following equation: EQU 2.pi.(f.sub.1 -f.sub.2) T+.phi.+.theta.=TAN.sup.-1 I/Q.
Note that any mismatches in overall amplitude or phase delay between the two quadrature channels will perturb the expected amplitude (I.sup.2 +Q.sup.2).sup.1/2 and phase relationship TAN.sup.-1 I/Q between input and output parameters. If the relative mismatch changes with input frequency, a distortion becomes apparent for cases of multi-frequency (wideband) input signals.
Conventional approaches to characterizing and aligning the channel responses for such quadrature detectors involve making many multipoint measurements of I and Q for a wide range of calibrated phase shifts of a single frequency input and then repeating these measurements for each frequency contained in the expected input bandwidth. The measurements are usually tabulated, related mathematically to the input signal, and then the components are manually tuned to correct for misalignment of phase and/or amplitude. Measurement and alignment are obviously complicated by the fact that a change in output amplitude of either I or Q at a given frequency may be caused by changes in either phase or amplitude or both in view of the amplitude (I.sup.2 +Q.sup.2).sup.1/2 and phase TAN.sup.-1 I/Q relationships noted previously.